Symmetric encryption algorithms, such as AES, Blowfish, 3DES, RC2, etc. are simply mathematical computations that transform input bytes to output bytes having the characteristics of random byte data (where each byte value 00 through FF is equally possible, and the bytes are seemingly random). There is no file format. In other words, encrypting a file w/ any of these symmetric algorithms does not result in output that is structured in any way (i.e. no headers, no identifying structure, etc.) Decryption is simply the reverse mathematical computation.

To tell if a particular file is encrypted or not, you'll have to use something that is known about the file type. Most file types are such that they begin with a recognizable first few bytes. For example, for tif:

A TIFF file begins with an 8-byte
image file header, containing the
following information: Bytes 0-1: The
byte order used within the file. Legal
values are: “II” (4949.H) “MM”
(4D4D.H) In the “II” format, byte
order is always from the least
significant byte to the most
significant byte, for both 16-bit and
32-bit integers This is called
little-endian byte order. In the “MM”
format, byte order is always from most
significant to least significant, for
both 16-bit and 32-bit integers. This
is called big-endian byte order. Bytes
2-3 An arbitrary but carefully chosen
number (42) that further identifies
the file as a TIFF file. The byte
order depends on the value of Bytes
0-1.

The same kind of thing can be said for many file types: Zip, PDF, etc..